Short users guide for SLUB¶
The basic philosophy of SLUB is very different from SLAB. SLAB requires rebuilding the kernel to activate debug options for all slab caches. SLUB always includes full debugging but it is off by default. SLUB can enable debugging only for selected slabs in order to avoid an impact on overall system performance which may make a bug more difficult to find.
In order to switch debugging on one can add an option slub_debug to the kernel command line. That will enable full debugging for all slabs.
Typically one would then use the slabinfo command to get statistical data and perform operation on the slabs. By default slabinfo only lists slabs that have data in them. See “slabinfo -h” for more options when running the command. slabinfo can be compiled with
gcc -o slabinfo tools/vm/slabinfo.c
Some of the modes of operation of slabinfo require that slub debugging be enabled on the command line. F.e. no tracking information will be available without debugging on and validation can only partially be performed if debugging was not switched on.
Some more sophisticated uses of slub_debug:¶
Parameters may be given to slub_debug . If none is specified then full debugging is enabled. Format:
slub_debug= Enable options for all slabs slub_debug=, Enable options only for select slabs
Possible debug options are:
F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS Sorry SLAB legacy issues) Z Red zoning P Poisoning (object and padding) U User tracking (free and alloc) T Trace (please only use on single slabs) A Toggle failslab filter mark for the cache O Switch debugging off for caches that would have caused higher minimum slab orders - Switch all debugging off (useful if the kernel is configured with CONFIG_SLUB_DEBUG_ON)
F.e. in order to boot just with sanity checks and red zoning one would specify:
Trying to find an issue in the dentry cache? Try:
to only enable debugging on the dentry cache.
Red zoning and tracking may realign the slab. We can just apply sanity checks to the dentry cache with:
Debugging options may require the minimum possible slab order to increase as a result of storing the metadata (for example, caches with PAGE_SIZE object sizes). This has a higher liklihood of resulting in slab allocation errors in low memory situations or if there’s high fragmentation of memory. To switch off debugging for such caches by default, use:
In case you forgot to enable debugging on the kernel command line: It is possible to enable debugging manually when the kernel is up. Look at the contents of:
Look at the writable files. Writing 1 to them will enable the corresponding debug option. All options can be set on a slab that does not contain objects. If the slab already contains objects then sanity checks and tracing may only be enabled. The other options may cause the realignment of objects.
Careful with tracing: It may spew out lots of information and never stop if used on the wrong slab.
Slab merging¶
If no debug options are specified then SLUB may merge similar slabs together in order to reduce overhead and increase cache hotness of objects. slabinfo -a displays which slabs were merged together.
Slab validation¶
SLUB can validate all object if the kernel was booted with slub_debug. In order to do so you must have the slabinfo tool. Then you can do
which will test all objects. Output will be generated to the syslog.
This also works in a more limited way if boot was without slab debug. In that case slabinfo -v simply tests all reachable objects. Usually these are in the cpu slabs and the partial slabs. Full slabs are not tracked by SLUB in a non debug situation.
Getting more performance¶
To some degree SLUB’s performance is limited by the need to take the list_lock once in a while to deal with partial slabs. That overhead is governed by the order of the allocation for each slab. The allocations can be influenced by kernel parameters:
slub_min_objects allows to specify how many objects must at least fit into one slab in order for the allocation order to be acceptable. In general slub will be able to perform this number of allocations on a slab without consulting centralized resources (list_lock) where contention may occur. slub_min_order specifies a minim order of slabs. A similar effect like slub_min_objects . slub_max_order specified the order at which slub_min_objects should no longer be checked. This is useful to avoid SLUB trying to generate super large order pages to fit slub_min_objects of a slab cache with large object sizes into one high order page. Setting command line parameter debug_guardpage_minorder=N (N > 0), forces setting slub_max_order to 0, what cause minimum possible order of slabs allocation.
SLUB Debug output¶
Here is a sample of slub debug output:
==================================================================== BUG kmalloc-8: Redzone overwritten -------------------------------------------------------------------- INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58 INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58 INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554 Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a . ZZZZZZZZ Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005 Redzone 0xc90f6d28: 00 cc cc cc . Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ [] dump_trace+0x63/0x1eb [] show_trace_log_lvl+0x1a/0x2f [] show_trace+0x12/0x14 [] dump_stack+0x16/0x18 [] object_err+0x143/0x14b [] check_object+0x66/0x234 [] __slab_free+0x239/0x384 [] kfree+0xa6/0xc6 [] get_modalias+0xb9/0xf5 [] dmi_dev_uevent+0x27/0x3c [] dev_uevent+0x1ad/0x1da [] kobject_uevent_env+0x20a/0x45b [] kobject_uevent+0xa/0xf [] store_uevent+0x4f/0x58 [] dev_attr_store+0x29/0x2f [] sysfs_write_file+0x16e/0x19c [] vfs_write+0xd1/0x15a [] sys_write+0x3d/0x72 [] sysenter_past_esp+0x5f/0x99 [] 0xb7f7b410 ======================= FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
If SLUB encounters a corrupted object (full detection requires the kernel to be booted with slub_debug) then the following output will be dumped into the syslog:
- Description of the problem encountered This will be a message in the system log starting with:
=============================================== BUG : ———————————————— INFO: — INFO: Slab INFO: Object INFO: Allocated in age= cpu= pid= INFO: Freed in age= cpu= pid=